Following the discovery in 2000 of the mechanisms underlying gecko adhesion, synthetic fibrillar dry adhesion has been an active research topic with over 100 publications on adhesive fabrication, modeling, etc. During this time, the number of robots that employ dry adhesion has been comparatively small, although several climbing, flying and space gripping examples are reported.
One difficulty with applying gecko-inspired dry adhesion in the real world is that it is difficult for the robot to know when it has achieved a reliable grip, especially on surfaces that may have defects or contamination. In contrast, the gecko has numerous mechanoreceptors in its toes, so it always knows whether it is attaching reliably. Indeed, a lack of adequate attachment at the forelimbs triggers automatic tail reflexes to prevent falling. Giving robots a similar ability to sense attachment quality will allow them to climb more reliably. Measuring the tail forces for a climbing robot can provide an estimate of adhesive forces at the feet, which aids climbing.
The strength of adhesion depends on the uniformity of contact between the adhesive and a surface. If a robot is climbing a window, it is often apparent by looking through the glass from the opposite side whether the robot's foot pads have attached properly; the color or brightness of firmly attached regions will be different from those of regions that are not. Unfortunately, this solution will not work for onboard sensing, as the robot only sees the window from one side. For testing of large adhesive tiles, a solution using optical emitters and detectors covered with a clear silicone layer followed by clear silicone fibrillar structures can determine what area is making contact.
In other work, an optical sensor with high spatial resolution, using frustrated total internal reflection, measures the positive and negative pressure distributions achieved by live geckos. However, this design is not practical for packaging into the foot of a small climbing or flying robot. It also does not suffice to measure the overall normal and tangential force at a robot foot. The reason is that these forces will increase up to the point of failure, with no warning of premature failure if the contact conditions are poor. Instead, one should measure a distribution of positive and negative pressures, as well as shear tractions, at each adhesive pad. To be practical, the sensor at each foot should also be small, thin, robust and lightweight, particularly for applications in micro air vehicles (MAVs).
What is needed is a sensor, which decouples normal and shear measurements and provides high sensitivity to shear loads.